The present invention relates to cementing operations, and more specifically, to cementing operations in subterranean formations that contain corrosive components.
Hydraulic cement compositions are commonly used in subterranean operations, particularly completion and remedial operations. For example, hydraulic cement compositions may be used in primary cementing operations whereby pipe strings, such as casings and liners, are cemented in wellbores. Hydraulic cement compositions may also be used in remedial cementing operations such as plugging highly permeable zones or fractures in wellbores, plugging cracks and holes in pipe strings, and the like.
A variety of hydraulic cement compositions have been used in conventional subterranean cementing operations with the most common cement compositions comprising Portland cement. However, Portland cement has drawbacks in its method of preparation, methods of implementation, and final set compositions. Portland cement is generally prepared by heating a mixture of raw materials comprising calcium oxide, silicon oxide, aluminum oxide, ferric oxide, and magnesium oxide in a kiln to approximately 1500° C. Thus, the energy requirements to produce Portland cement are quite high, and heat loss during production can further cause actual energy requirements to be even greater. In addition, Portland cement manufacturing process produces about 5% of the total global anthropogenic CO2. This makes for an expensive production method with a high carbon footprint. The manufacturing process of Portland cements also has batch-to-batch variations that may yield unpredictable results when applied in a wellbore.
In addition to manufacturing drawbacks, the implementation of Portland cements in subterranean formations also has drawbacks. Salts, particularly multivalent salts, often cause issues during the pumping and installation of a Portland cement. For example, when exposed to magnesium or calcium salts, Portland cement slurries are known to rapidly viscosify to a point that the cement is no longer pumpable. In subterranean formations, magnesium and calcium salts may be encountered in brines, evaporite minerals, and salt domes. To mitigate this effect, engineers may add scale inhibitors, chelating agents, or other additives to a treatment fluid containing Portland cement. However, this method is typically restricted because of very high material and installation costs.
Further, once the Portland cements are set within a wellbore, corrosive components, like salts, carbonic acid, and hydrogen sulfide, found within some subterranean formations may cause failure of Portland cement structure. As used herein, the term “corrosive” refers to a substance that destroys or irreversibly damages another surface or substance with which it comes into contact. For example, loss of metal due to chemical or electrochemical reactions is a commonly known form of corrosion. Corrosion rates may vary depending on the time, temperature, corrosive component, pH, and other physical and chemical variable. For example as shown in the chemical reaction below, dissolved carbon dioxide and carbonic acid can attack Portland cements by converting calcium hydroxide to the more stable calcium carbonate and/or calcium bicarbonate. First, dissolved carbon dioxide converts to carbonic acid thereby lowering the local pH. The rate of conversion may depend on temperature, partial pressure of carbon dioxide, and salt concentration. Second, carbonation of the Portland cement occurs which may cause (a) densification leading to increased hardness and reduced permeability thereby decreasing CO2 diffusion and (b) volume expansion of up to 6%, which may lead to development of micro to macro cracks in extreme cases. Both of these results may be due to an increase in mass (from chemical consumption of CO2) within the volume defined by the solid set cement matrix. Finally, the long-term phenomenon of dissolution of CaCO3 may occur when the cement is surrounded by water containing dissolved CO2 for extended periods of time. Dissolution of CaCO3 may increase porosity and/or permeability thereby decreasing overall mechanical strength. Decreased cement integrity may lead to inefficient zonal isolation and in extreme cases complete failure of the cement composition.
1) Formation of carbonic acid:CO2+H2O→H2CO3 
2) Carbonation of Portlandite and/or cement hydrates:Ca(OH)2+H2CO3→CaCO3+2H2OC—S—H and/or crystalline phases+H2CO3→SiO2(gel)+CaCO3+H2O
3) Dissolution of CaCO3 (long-term effect):CaCO3+H2CO3→Ca(HCO3)2 
Carbon dioxide and/or carbonic acid corrosion may, through the above mechanisms, lead to decreased strength of a Portland cement composition ultimately causing cracking and failure of a subterranean cement structure. This corrosion may be of greater concern depending on the characteristics of the subterranean cement structure. For example, increased surface area and/or increased permeability of the cement structure to water, as may be the case in a foamed cement structure, may dramatically increase the rate at which the structure corrodes causing a shorter usable life.
By a similar mechanism, sulfuric acid may cause Portland cement corrosion. Sulfuric acid corrosion may be magnified if the wellbore contains sulfate salts and/or bacteria that metabolize hydrogen sulfide and/or sulfur to sulfuric acid.
Additionally, hydrogen sulfide may cause significant Portland cement deterioration. Hydrogen sulfide in the presence of water converts to HS− and/or S2− that reacts with the calcium hydroxide and transition metal oxide containing components in Portland cement to form calcium sulfide and transition metal sulfide. For example, iron containing components, such as calcium ferroaluminate (C4AF) (generally present from 8-13% Portland cement), may react with hydrogen sulfide by the reaction:C4AF or FeXOy+H2S→FeS2+H2(gas)
To mitigate the corrosive damage, engineers use other cementitious compositions to replace at least some of the Portland cement in subterranean operations. This can be effective for formation with moderate corrosive capacity. However, it would be advantageous to have a cementitious composition essentially free of Portland cement for use in subterranean formations with high corrosive capacity or compounding corrosive components.